The present invention generally relates to radiocommunication systems and, more specifically, to an apparatus and associated method for efficiently controlling power amplifiers within radio transmitters in cellular systems.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmissions. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of personal communication networks (PCN) (employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make and receive calls in the home, office, street, car, etc.), would be provided by the cellular carriers using the next generation digital cellular system infrastructure and the cellular frequencies. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is most commonly achieved using frequency division multiple access (FDMA) and time division multiple access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel radios.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster bit rate which increases the amount of occupied spectrum proportionally.
With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, it is desirable to avoid the case where two potentially interfering signals occupy the same frequency at the same time. In contrast, code division multiple access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other.
For all such systems, but especially CDMA systems, power control is an important technique for balancing the desire to provide an end user with a sufficiently strong signal while at the same time not causing too much interference to other users. Various types of power control techniques exist, but many involve being able to adjust the power at which a signal is being transmitted by predefined increments.
Power amplifiers (PAs) are widely utilized in radio transmitters in order to generate amplified signals to be transmitted. The aforedescribed power control techniques may control a transmitter's power amplifier to amplify RF signals at one of several distinct power levels. The more accurate the
in the amplification of these signals, the more efficient the transmitter operation.
In order for a power amplifier to operate efficiently, a smooth and accurate power level ramping between power levels, as well as ramping-up upon powering the PA on and ramping-down upon powering the PA down is desirable. A PA's power control circuit, determines a sequence of power level values the PA is to use when ramping up to the next power level or ramping down from a previous power level. The smoother the ramping sequence used by the PA in operation, the more accurate the PA operation and the less noise and other problems are caused thereby. In most systems, the optimal sequence of power ramping values are predefined and specified for the air interface. These values are stored in a memory device associated with the control circuit of the PA, as described with respect to FIG. 1, below. Sequences for each power level can be stored in the memory device and accessed when required to control the PA. However, even when the memory device of a control circuit is loaded with the most efficient ramp-up and ramp-down values, there still exists a need to improve the resolution of the analog control signals sent to the power amplifiers.
The signal processing normally occurring between the output of the PA power control circuit and the PA includes conversion of the digital value determined in the control section to an analog value and removal of transient signals through the use of a pass filter, which removes the transient signal energy without affecting lower frequencies of the signal being sent to the PA.
Sigma-delta modulation employed within digital-to-analog (D-A) converters is known to have several advantageous effects with respect to digital signals. Initially, sigma-delta techniques tend to provide better resolution than conventional D-A converters which were fabricated using analog components. A sigma-delta modulator also has the effect of moving noise out of the low frequencies of a signal. A further description of the uses and implementation of a sigma delta modulator per se, albeit not in the context of PA control, is discussed in U.S. Pat. No. 5,117,234, incorporated herein by reference.
One drawback of this type of implementation is that a sigma delta modulator is is quite complex. Thus, there is a need for a system that can utilize the advantages provided by a sigma-delta modulated digital-to-analog converter in a PA control circuit but at a reduced cost and complexity level.